a. Field of Invention
This invention pertains to an air bag restraint system for vehicles, and more particularly to a system with venting means to allow the air bag to collapse in a controlled manner.
Automobile collisions involve vehicle impacts that result in abrupt changes in the vehicle's velocity. Occupants riding within a vehicle must also sustain these velocity changes if they are to remain within the occupant compartment. However, the manner and timing in which they will experience them is dependent upon the way in which the occupant is restrained within the vehicle.
Technically, unless ejected, the occupant is always "restrained" in some way or another. For frontal impacts, restraint will be either by a seat belt, air bag, or violent contact with the steering wheel, windshield or instrument panel. In other words, sooner or later, the occupant's velocity must be equalized to that of the vehicle, resulting in a change in absolute velocity relative to the ground.
In terms of energy, the occupant's velocity change equates to change in the magnitude of his kinetic energy. The relationship between velocity and kinetic energy is strictly defined and given by well-known mathematical formulas. The purpose of any occupant restraint system then is to help achieve this change in energy with a minimum of traumatic force.
In terms of physics, the energy change is achieved by applying a restraining force over a given distance. In simple terms, the less distance over which the force is applied, the higher the applied force will have to be in order to achieve the given energy change. The minimization of restraint force is therefore achieved by maximization of the "over ground" distance over which the force is applied.
In a barrier collision, for example, the theoretical maximum over ground distance available to the occupant includes the following: the distance that the occupant compartment travels as the front end of the vehicle structure is deforming plus the distance within the compartment that the occupant can traverse before unwanted contact with a "hard" structural surface (i.e. steering wheel, windshield, instrument panel, etc.).
Because the over ground distance includes the crush of the vehicle's structure, it is important to begin restraining the occupant as soon into the crash event as possible. In this way, the occupant will be using the deformation of the vehicle to help dissipate his energy.
In an ideal situation, a constant restraint load is applied at the instant the impact begins and is such that the occupant utilizes all the available distance within the occupant compartment. Such a force would be the minimum required to dissipate the occupant's energy.
In the worst case, no restraint force is applied until the vehicle is fully deformed and come to rest, and the occupant has completely traversed the compartment. At this point the occupant will then impact the steering wheel, windshield and/or instrument panel. Depending upon the compliance of these components (which, in general are not very compliant and also tend to exert localized loading), the occupant will sustain his energy change by way of a very high force exerted over the relatively short distance these components will yield.
What can realistically be achieved lies somewhere in between these two extremes. Seat belts, one would think, directly and immediately apply the restraint load to the occupant. In reality, the compliance of the belt and occupant, slack and spoolout from the retractor, and the belt geometry relative to the occupant serve to delay the onset of significant restraint force until well into the vehicle impact event.
Air bag systems also involve a delay in the application of restraint load. The sensing of the crash severity and decision to deploy in conjunction with the time required to deploy and fill the bag constitute the delay associated with inflatable restraint systems.
As stated previously, the purpose of a restraint system is to minimize the traumatic force applied to achieve the occupant's required change in velocity . Strict minimization of the force magnitude is not sufficient in itself. The application of the force should be done in such a way as to minimize the trauma it incurs.
In this respect, air bags excel over all other currently available restraint systems. In comparison to seat belts, which essentially exert a line force across the pelvis and diagonally across the torso, an air bag distributes the restraint load over the entire upper torso and face. The air bag's ability to spread the restraint force over a large area of the occupant's body significantly reduces the potential for trauma.
However, once the air bag is deployed, it becomes relatively rigid so that a secondary collision between the air bag and the occupant may occur which has a much lower impact then without the air bag, but may still be strong enough to cause injuries. In fact under certain conditions, the occupant may rebound hard enough to lose contact with the bag.
b. Description of the Prior Art
As a solution to this problem, air bags have been suggested which have one or more relatively large vent holes for venting the gas from the air bag at a predetermined rate. This solution is feasible if a clean gas (i.e. a gas without particulate matter) is used for pressurizing the bag. It has been found that the pressurized gas is preferably produced by reactions of chemicals because prior to the reactions, the solid and/or liquid reactants can be stored in a very small space. This consideration is especially important for air bags which are mounted in a steering column. However, gas produced by such chemical means includes a large concentration of particulate matter and therefore it is unacceptable in air bags with venting holes described above because the vented gas escapes into the passenger compartment together with the particulate matter with detrimental effects on the occupants.